I occasionally make maps with Bryce and profantasy fractal terrains. I posted a thread about one of the procedures I use here:
I have used the method set out in the Bryce tutorial before, but I found it difficult to control. What I do instead is make a 'landscape' mesh by deforming a grid using a 3d modelling app which allows displacement painting (I use Silo, but there are many others) and import it into Bryce as an .obj file.
Then I use the top down camera in Bryce and render it using the distance mask which effectively makes the height map which you can paste into a new terrain and tweak using the Terrain editing tools.